The present invention relates to apparatus for the dissipation of thermal energy generated by semiconductor devices, and more particularly, it relates to heat dissipator apparatus designed to connect directly to semiconductor package structures in order to maximize heat transfer to the surrounding environment.
Many semiconductor devices generate heat during operation which must be dissipated. In many devices the heat generated is sufficiently self dissipated by the package and the leads. The goal of attaching a heat dissipator or heat sink to an electronics package when self heat dissipation is not enough, is to keep the semiconductor junction temperatures within that package low enough to assure reliability. In general, this is accomplished by attaching a heat sink as thermally close to the chip (or junction) of the device as possible. Such heat sinks can be mounted to a device package and comprise a bulk mass of thermally conductive material such as copper, aluminum or the like, with a large surface area which dissipates the heat into the surrounding environment. Such heat sinks can be extruded or made of stamped sheet metal, and can include heat dissipating fins or wings for extra heat dissipation surface area.
Many heat generating integrated circuit (IC) packages are provided with a metal tab or slug which is thermally coupled to the junctions, extends outwardly from the encapsulation body, and provides the main path for heat flow out of the device to the attached heat sink. One can attach the heat sink, via a clip or screw, directly to this metal slug, or through an insulating wafer using a heat conductive dielectric compound if electrical isolation is required.
Integrated circuits are produced in a variety of protective packages. A common package is the dual-in-line package, or DIP, in which the integrated circuit chip is encapsulated in a sealed rectangular plastic body of dielectric material. DIPs however, are difficult to heat sink because often there is no metal slug/tab to which one can attach a heat sink. Most often the heat generated in a DIP leaves the package by conduction through the leads with some heat dissipating by radiation and convection from the body. That the entire package is encapsulated in plastic, makes the body of the package an inefficient place to attach a heat sink, although this can be done.
A plurality of solderable metallic terminals, called pins or leads, are located at the long sides of the DIP package. In the high powered DIP package of the exemplary embodiment, the plurality of the pins on one side are electrically connected to the integrated circuit chip inside the body, thereby permitting electrical connections to be made to the chip circuit. The plurality of pins on the other side of the body are in direct physical contact with a metallic ground plane on the bottom of the chip, and heat conducting leads are used to conduct heat from the chip to the external environment. A heat dissipator can be attached, i.e., soldered, to the heat-conducting pins and thereby further aid in the conduction of heat away from the integrated circuit chip, such as in U.S. Pat. No. 4,254,447 of Griffis. In the prior art, heat sinking has been done by soldering the heat conducting leads to a ground plane formed from copper foil on one or both sides of the printed circuit board (PCB), as shown in FIGS. 1 and 2 and discussed more fully below. Further, a unitary heat sink has been soldered to the PCB copper foil ground plane proximal to the IC, as shown in FIG. 3, also discussed more fully below. For the latter arrangement, heat would flow through the heat conducting leads, through the first solder junction between the leads and the copper foil, the copper foil, a second solder junction from the copper foil to the heat sink, and into the heat sink. This long path and plurality of junctions with each junction having a thermal resistance, can result in higher temperatures.
When a heat sink is used, it is desirable to use a heat dissipator that is economical to manufacture, easily mounted in conjunction with the IC package, and maximizes the quantity of heat dissipated.
It is desirable that a heat sink should be attached as thermally close to the semiconductor chip as practicable to make an efficient path for the heat to flow to the heat sink. The present invention provides such an efficient means of connecting the heat conducting leads of a DIP package to a heat sink. With the present heat sink design, the heat conducting leads of the device are inserted into receiving apertures in the heat sink. The receiving apertures are in register with the heat conducting pins, i.e., being of the right spacing and size, and are disposed in a xe2x80x9cUxe2x80x9d shaped contoured portion which doubles back on itself, forming a channel or reservoir.
During assembly, the contoured part of the heat sink is mounted onto the PCB, protruding below the board surface, and the integrated circuit device is installed on the PCB with the heat conducting pins installed into respective apertures in the heat sink channel. The leads, PCB, and contoured portion of the heat sink are then soldered together.
Because the heat sink is soldered directly to the heat sinking leads, the heat sink provides maximal thermal conduction from the semiconductor device by reducing the path length of heat conduction between the device and the heat sink. This is enhanced by the leads each being soldered in their own apertures which are spaced apart, thus distributing the heat flowing to the heat sink.
Additionally, the heat sink channel forms a collector or catch basin for the accumulation and retention of solder when the assembly and PCB are soldered. This added bulk mass of heat conducting solder remaining in the channel in intimate contact with the heat conducting leads further enhances the flow of heat out from the IC chip.